U.S. Pat. No. 3,914,771 issued on Oct. 21, 1975, to Lunde et al, shows an electrographic printer which selectively transfers toner particles directly onto a receiver. It incorporates a magnetic brush assembly 10, which is shown in FIG. 1. This Prior Art brush assembly includes an imaging head 12 that is located within a slot on the exterior surface of the shell 14, and is substantially flush with the surface of the shell. It also includes a rotatable multi-pole magnet 16, mounted within the shell 14. FIG. 2. shows a second arrangement wherein the imaging head 12 is mounted directly upon the shell 14. The first arrangement is preferred since it eliminates the need for a ramp 19 to deliver toner particles to the imaging head 12, and provides a channel for electrical connections to the head. This slotted method of assembly, while easing the complexity of attaching the imaging head 12 to the shell 14, also creates a problem in manufacturing when the shell is conductive. The act of machining the slot 18, shown in FIG. 3, requires that the outside diameter of the shell 14 be increased by at least the thickness of imaging head 12 to maintain the structural integrity of the shell. The increased thickness of the metal, however, decreases the electromechanical efficiency of the magnetic brush 10. The rotation of the multi-pole magnet 16, within shell 14 by drive motor 20, generates eddy currents within the shell 14.
In applications using dual-component developers the shell of the magnetic brush must be conductive in order to minimize tribo-electrical charging of the developer during image development and also allow for the application of a bias voltage for adjusting image and background densities to acceptable values. In conventional electrophotography the magnetic brush's shell has nothing mounted on its surface and therefore can be made as thin as possible consistent with the requirement of a constant gap between the cylindrical permanent magnet core and the outer surface of the shell. However, if the shell is made thick enough to accommodate a printhead as shown in FIG. 4, when the core magnet is rotated to transport the developer, eddy currents 17 are generated along the length of the shell 14. The eddy currents dissipate power by resistive heating of the shell and react with the field of magnet 16 to generate a drag torque which acts in opposition to the drive torque thereby requiring significant additional power applied to the motor to rotate the core permanent magnet. The eddy currents 17 form closed loops that circulate along the length of the shell 14. This additional power not only degrades the motor performance but results in additional heat which can damage the motor as well as effect the physical and electrical properties of the developer.